The invention relates generally to modulatable devices and more specifically to a method and a system for remotely setting a bias point of the modulator.
External modulation of a continuous wave source using an optical modulator such as a Mach Zehnder modulator is a widely used method in analog optical links. Optical modulators typically have a nonlinear transfer function for the conversion of electrical modulation into optical modulation. In order to maintain a link with low distortion, the modulator is biased at a certain point in the transfer function, typically known as the bias point.
The optimal bias point may drift over time, for various reasons such as aging of the modulator, temperature, and other material related phenomena. To maintain the optimal bias point, this drift must be tracked and then the applied bias adjusted to reflect the new state of the transfer function. This requires some amount of external control electronics to process information from the modulator. In some applications, such as magnetic resonance imaging (MRI), where it may be desirable to place the modulator at the receive coils, there is limited space, limited electrical power and the environment may be harmful to electronics due to interference effects.
In addition, in MRI applications, the modulator bias is required to be set before each scan of a patient. When the bias is set, the scanning may be done within a time frame where the bias remains at the optimum level. Other applications may be in remote sensing, where a low distortion link is required, but only for a duration that is sufficiently short so that the bias does not drift from its optimum value during the time it is in operation.
Typically, the bias point control has been addressed by use of a pilot tone. The pilot tone is mixed with the modulation signal and the bias point is adjusted through a control loop that uses harmonics of the pilot tone or phase differentials of the pilot tone. However, mixing a pilot tone with the modulation signal can introduce distortion to the modulation signal, limiting the dynamic range of the link. In addition, a stable and accurate local clock source is required to generate the pilot tone, which may not always be available.
Therefore, there is a need to remotely monitor and control the bias of a modulatable device used in applications where minimum distortion and high dynamic range is desired.